Computer aided tomography systems derive images by placing the subject between a source and one or more radiation sensors, wherein a source of radiation and radiation sensors move together in one or more rotational or lateral axes. The computer aided tomography systems produce the image by indirect means. Specifically, a multiplicity of x-ray readings are taken of the subject which in themselves do not directly characterize the elements within the subject as known and readable to the human observer. The computer then interprets the multiplicity of x-ray readings taken together in a particular manner in which a readable image is created to define the subject of interest. A subgroup of tomography systems of present interest is known as continuous wave fan beam tomography systems. The term "continuous wave fan beam" specifies the radiation source to be continually emitting radiation in a beam pattern resembling a sector of a circle. As part of this type of system, there are typically several hundred radiation sensors in the path of the fan beam on the opposite side of the subject to receive the x-ray radiation as attenuated along defined path lines through the subject. The sensors form an arcuate segment of sufficient length to intercept the entire radiation beam transmitted along a path through and absorbed by the subject from the radiation source and generate individual output signals. In prior fan beam tomography systems simultaneous acquisition of all x-ray readings for each gantry position are believed to be the most preferred manner in which to provide the necessary mathematical basis for tomographic image (re)construction. The individual radiation sensor output signals are therefore (in prior systems) each initially processed by individual resettable integrators, known collectively as the "integrate-and-dump" technique, which are then sampled and stored by subsequent system elements. According to the continuous wave fan beam tomography systems and included techniques of the prior art, it is believed that the best estimate of the radiation attenuation along each defined path can be derived from the individual outputs of integrate-and-dump circuits which are designed to collect as much detector output signals, and therefore x-ray radiation signals, as possible.
Another aspect of prior art, relating to computer-aided continuous wave fan beam tomography systems has been the great expense of the required associated auxiliary circuitry, in addition to the several hundred precision resettable integrators. In general, auxiliary transfer circuitry has been required for each input channel to rapidly receive, upon a transfer command, the accumulated integration values, allowing the aforesaid input integrators to be rapidly reset at the end of an integration period and be allowed to continue to integrate during the next integration time interval.
A universal goal in tomography systems is to reduce the radiation dosage. The dosage requirements are generally defined by the detector sensitivity to x-ray radiation, the radiation detector geometry and the subsequent signal processing apparatus. In the prior art, it is assumed that collecting as much radiation signal as possible (with a resettable integrator for each radiation detector) was the best way of minimizing the radiation dosage. However, in spite of system improvements according to prior art teachings, the dosage required still remains relatively high, thus limiting the number of examinations any one subject may undergo.